It is already known (DE OS 37 38 845 A1, DE 34 11 536 C2) to limit a target to only the area that is to be sputtered or ablated. This is achieved advantageously in the case of the interpolar target cathode by having the target alone at potential. Furthermore, in the case of large cathodes used in glass coating apparatus it is achieved by covering with masks those areas of the target which are not sputtered or ablated. This, however, has the disadvantage that these masks also become coated and these portions of the coating can flake off, fall onto the substrate, and thus render it unusable. In the case of relatively large cathodes, for example those of great length, such masks can become relatively easily distorted due to their instability, so that short-circuiting can occur.
If reactive sputtering processes are performed by cathode sputtering using a noble gas atmosphere and metal is sputtered and oxides or nitrides are to be formed, a reactive gas is fed to this atmosphere. The reaction products thus obtained are normally deposited not only on the substrate but also on the target surface. If such reaction products are highly insulating, such as silicon oxide or aluminum nitride, the charging up of this surface results in dielectric breakdowns, i.e., so-called flashover (arcing) and therefore also in stability problems. In highly reactive processes, these stability variations cause the layers to become irregular and the arcing causes particles to flake off from the target surface and then deposit themselves on the substrate. Especially in electronics, if insulating coatings are involved, this renders the substrate unusable. In the production of optical coatings, irregularly applied coatings can result in greater light scattering in the coating.